Conventionally, it is known to conduct “noise log” operations in oil/gas wells. The noise log tool is an instrument sensitive to acoustic signals and is run into a well on the end of an electrical cable attached to, or contained within a wireline, in order to move it past zones of interest.
Accordingly, as the noise log tool is drawn past the zones of interest, anomalies or differences from a baseline log (i.e. a log already conducted) can be used to infer information on the flow of the production fluids (i.e. the oil and/or gas) passing up the well from the production zones. Such conventional noise log technology, however, provides only a “one-shot” survey, in that it only provides information when the noise log tool is actually run into the well and therefore cannot provide a permanent monitoring system since the noise log tool cannot reside permanently within the well. Furthermore, the measurements provided by the noise log tool may be affected by the very presence of the noise tool in the flow.
It is also known to use optical fibers deployed in oil and/or gas wells, where the optical fibers are provided with a number of Fiber Bragg Gratings (FBGs). By way of background, FBGs have been in use for many years in the telecommunication industry and have been used more recently in the downhole oil and gas industry. Hitherto, the FBGs have been used to act as sensors to monitor seismic activity in the oil and/or gas reservoir; in other words, they listen for movement such as cracking in the rock of the reservoir. An article entitled “Downhole Monitoring: Listening with Light” published in April 2003 on pages 26 to 30 in Offshore Engineer publication discusses such known use of FBGs.